Pa. FOSSILS -marine cretaceous? Help I.D.

I know next to nothing about fossils, this is a new interest for me. I have an Enormous slab of rock composed of almost entirely of ancient sea life. These are pics. of the rock from a few different angles. The The first pic. is what I had dug out of a piece which has broken off. I was given this by my Uncle and was told there are many Trilobite remains in it along with various other types of sea life. It looks like he or someone had extracted many others from this as well because of all of the empty holes on the top. I don't think the one I extracted and have shown here(single fossil solo) is a Trilobite and I would like to know what it is and what others the Large slab may contain. The fossils came from right here in Pa. , North Central Pa. at what is referred to as "The Pennsylvania Grand Canyon" . Any help is always appreciated.Thanks!